Article 155: What Spain's 'nuclear option' really means

In plain English, the article means that if a self-governing community, like Catalonia, has acted in any way to undermine the interests of Spain, the national government will "take all measures necessary" to force it to meet its obligations to the state.

Here is the actual wording of Article 155 in the Spanish constitution:

"If a self-governing community does not fulfill the obligations imposed upon it by the constitution or other laws, or acts in a way that is seriously prejudicial to the general interest of Spain, the government, after having lodged a complaint with the president of the self-governing community and failed to receive satisfaction therefore, may, following approval granted by the overall majority of the senate, take all measures necessary to compel the community to meet said obligations, or to protect the above mentioned general interest."